1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a facsimile machine or a printer, and more specifically, to a toner supply device for supplying toner to an electrostatic transport member which carries the toner to a developing position with an electrostatic force and a developing device including the toner supply device.
2. Related Background Art
An image forming apparatus such as a copying machine, a facsimile machine, or a printer is constructed to form an electrostatic latent image on a latent image carrying member such as a photosensitive member, to visualize the electrostatic latent image as a toner image using a one-component developer composed of toner, or a two-component developer composed of toner and carrier, and to directly transfer the toner image to a recording medium such as a transfer paper or indirectly transfer the toner image to a transfer paper via an intermediate transfer member such as an intermediate transfer belt. An example of such an image forming apparatus is disclosed in JP 9-197781 A and JP 9-329947 A. In the prior art image forming apparatus, toner on a surface of a developer carrying member such as a developing roller is transported to a developing to develop an electrostatic latent image on a photosensitive member.
However, there is a problem in the image forming apparatus that that the toner adheres and sticks to the surface of one of the photosensitive member and the developing roller due to friction therebetween to adversely affect an image in quality. In addition, the toner at an developing position is electrostatically moved by a potential difference between the surface of the developing roller and the electrostatic latent image on the photosensitive member. In this case, this potential difference has to be considerably large because, prior to electrostatically moving the toner, the toner should be applied with a force which can overcome an adhesive force between the toner and the developing roller due to a Van der Waals force, an image force so that the toner is released from the adhered state.
On the other hand, an image forming apparatus for developing a toner image without using a developer carrying member such as a developing roller is disclosed in JP2002-341656A. The prior art image forming apparatus is constructed to transport toner to a developing position in an Electrostatic Transport & Hopping (ETH) phenomenon which occurs on a surface of an electrostatic transport substrate on which plural electrodes are disposed at a predetermined pitch. In the ETH phenomenon, energy in a phase shift electric field acting on powder or fine particles is converted into mechanical energy so that the fine particles themselves are dynamically varied. In other words, the ETH phenomenon causes the fine particles to jump with a component in a traveling direction owing to the phase shift electric field on the surface of the electrostatic transport substrate so that both the movement or carrying in a direction along the surface of the substrate and the movement or hopping in a direction perpendicular to the surface of the substrate are performed. Therefore, when the toner on the surface of the electrostatic transport substrate is carried to the developing position while eliminating the adhesive force in the ETH phenomenon, a desirable low potential development, which cannot be expected in the developing device of the structure using the developer carrying member, can be realized. For example, it is also possible to selectively adhere toner to an electrostatic latent image which has a potential difference of only several tens of volts betoween an image portion and a non-image portion of the electrostatic latent image.
In the developing device of the prior art image forming apparatus, however, a sufficiently charged toner cannot be supplied to the electrostatic transport substrate, with the result that a satisfactory development can not be performed. Usually, toner can be charged due to friction by an agitator in a toner hopper or a doctor blade when the toner is drawn up to a surface of a charging roller from the toner hopper. However, there is a problem that the toner cannot be charged sufficiently by the agitator or the doctor blade with such a low friction.
To solve the problem, the toner may be charged by a corona discharging device. However, it is likely that a large quantity of toner is scattered by impact caused. by the corona discharge. In addition, since ozone is generated by the corona discharge, this is not preferable for the environment.
Moreover, it is to be noted that a developer should be supplied to a photosensitive member such that the developer is distributed uniformly over an entire length of the photosensitive member. Thus, conventionally, a toner agitating and transport screw device having a spiral screw formed on an outer periphery of a main shaft is used for agitating and transporting toner to a toner supply device. Examples of such a toner agitating and transport screw device are disclosed in JP 2001-331024 A, JP 2001-331025 A, and JP 10-221937 A, or JP 3319844 B.
However, as an amount of toner consumption per unit time in a developing device of a recent image forming apparatus increases in accordance with an increase in copying or speed, it has been demanded to supply toner rapidly and mix and agitate the toner sufficiently in short time to increase a charging amount of the toner to a suitable level for development. However, the prior art agitating and transport screw device having a screw-like vane cannot meet such a demand for the developing device of the recent image forming apparatus.